Selected genes in Drosophila melanogaster are being studied in an effort to understand their organization and regulation through the analysis of mutations that upset regulatory functions. A locus of particular interest is white, which controls one of the steps in the pigmentation of the eyes, malpighlian tubules and testis sheath in Drosophila. A large number of mutations, some of which appear to upset the regulation of the locus, have been genetically characterized. The molecular analysis of many of these mutants has also been done, showing that a large proportion of spontaneous changes result from the insertion/deletion of transposable elements. Two aspects of transposon-induced changes at white are being studied. One deals with modifications of regulation brought about by insertion of the transposon BEL. In one class of changes, white function is expressed as a mosaic of normal and mutant pigmented patches in clonal patterns in the eye. Another class exhibits mosaicism but in nonclonal, nonautonomous patterns. Both types of mutations are related by descent, having their origin in the insertion, excision or transposition associated with the BEL transposon. The transposon has been molecularly cloned and mapped by restriction endonucleases and a number of the mutations it caused have been characterized molecularly. Other transposon-induced modifications at white locus are transposon involvements in asymetrical pairing and exchange that result in duplication/deficiency crossover products. Analysis of one set of such products suggests that copia and B104, though lacking any detectable homology, have undergone an exchange to produce a chimeric copia-B104 sequence at the junction of the exchange site. This unusual event is being further investigated by sequencing the junction segment.